Variable resistance memory devices, also known as resistance variable memory elements, utilizing chalcogenides have been used as semi-volatile and non-volatile random access memory devices. U.S. Pat. No. 6,348,365 entitled “PCRAM Cell Manufacturing” discloses a typical chalcogenide variable resistance memory element. A typical chalcogenide variable resistance memory element often incorporates a conductive material into a chalcogenide glass. The conductive material may typically be silver and/or copper. The resistance of the chalcogenide glass can be “programmed” to have two logic states, a high resistant state and a low resistant state. The memory element may be in the low resistant state during a write operation and in the high resistant state during a read operation. In operation, a voltage potential may be applied across the chalcogenide glass to “program” the memory element to the low resistant state permitting a write operation to the memory element. A voltage potential having a lesser magnitude may then be applied to the memory element to permit a read operation on the device. A chalcogenide variable resistance memory element can function as a semi- or non-volatile variable resistance memory having at least two resistance states.
The low resistance state of a chalcogenide variable resistance memory element may remain intact for days or weeks after the voltage potentials are removed from the device. Such material can be returned to its high resistance state by applying a reverse voltage potential between the electrodes of at least the same order of magnitude as used to write the element to the low resistance state. Again, the highly resistive state of the device may be maintained once the voltage potential is removed. This way, such a device can function, for example, as a variable resistance memory element having two resistance states, which can define two logic states.
One preferred variable resistance material comprises a chalcogenide glass. A specific example is germanium-selenide (GexSe100-x) comprising silver (Ag). One method of providing silver to the germanium-selenide composition is to initially form a germanium-selenide glass and then deposit a thin layer of silver upon the glass, for example by sputtering, physical vapor deposition, or other known techniques in the art. The layer of silver is irradiated, preferably with electromagnetic energy at a wavelength less than 600 nanometers, so that the energy passes through the silver and to the silver/glass interface, to break a chalcogenide bond of the chalcogenide material such that the glass is doped or photodoped with silver. Silver may also be provided to the glass by processing the glass with silver, as in the case of a silver-germanium-selenide glass co-sputtered with Ag. Another method for providing metal to the glass is to provide a layer of silver-selenide on a germanium-selenide glass.
U.S. Pat. No. 7,304,368 discloses a variable resistance device that has various layers including a chalcogenide glass layer and a tin-chalcogenide layer between two electrodes. Upon the application of a potential across the variable resistance device, it is believed that tin from the tin-chalcogenide layer may be incorporated into the chalcogenide glass layer to form a conductive channel through the chalcogenide glass layer, which causes a detectible resistance change across the device. Variable resistance devices, such as the device disclosed in U.S. Pat. No. 7,304,368, which is hereby incorporated by reference, may be difficult to tune in the high resistance programming range, and preferentially operate in a binary mode between high and low resistances. Other drawbacks of current devices may also exist.
Accordingly, there is also a need for variable resistance device that is easier to tune in the high resistance programming range, and for variable resistance memory devices that are stable and repeatable in operation, and resistant to silver migration during thermal stresses such as those present in higher temperature device operation and in some back-end-of-line processes (as long as the temperature remains below the chalcogenide glass transition temperature).